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INTHODUCTORY  ESSAY.                                178
pal vegetation occurs at the commencement of spring, when
the melting snow supplies abundant moisture to small an-
nual plants, which run their course with great rapidity, and
arc speedily shrivelled up by a scorching sun.
As respects climate, we have therefore two different systems
of division of the Himalaya:1, into the tropical, temperate,
and alpine zones | and 2, into the exterior or rainy, the inte-
rior or intermediate, and the Tibetan or arid Himalaya.
The term tropical is not strictly applicable to any part of
the chain, which is nowhere within the tropics, but we find it
convenient to adopt it, and, the vegetation being strictly tro-
pical, it can, we think, lead to no inconvenience; while the
only word which could be substituted, namely subtropical, is
required to express the transition from the vegetation of the
base to that of the temperate zone. There are of course no
strict lines of demarcation between the three zones first enu-
merated ; but they are sufficient to express the three promi-
nent changes in the vegetation which correspond to those
observable in passing from the equator towards the poles, and
on the whole are sufficiently distinct to be readily recognis-
able.
In the extreme west 'the tropical belt rises to about 4000
feet, and as we advance eastward its elevation gradually in-
creases. In Kumaon it is 5000 feet, and in Nipal rather
higher. In the permanently humid country ta the eastward
it rises still higher, tropical vegetation being found as high as
7000 feet; but the equable nature of the climate masks the
effect, and carries many temperate plants much lower than
that level. The alpine zone may be said to commence at the
upper limit of trees, which varies from 12,000 feet in the
extreme west to nearly 13,000 feet in the eastern Himalaya.
A. number of trees and shrubs which are peculiar to the
higher part of the temperate zone, we shall generally charac-
terize as subalpine.
The division of the Himalaya into exterior, interior, and
Tibetan, corresponds in the temperate zone to very marked